Styrene monomer is typically produced from ethylbenzene at temperatures from about 600° C. and higher by vapor phase catalytic dehydrogenation in the presence of steam. Known catalysts include palladium oxide, platinum metal, molybdenum-bismuth oxide and oxides of copper, zinc, arsenic, antimony, chromium, iron, and cobalt.
The hot gaseous effluent from the dehydrogenator contains primarily styrene, hydrogen, unreacted ethylbenzene, divinylbenzene water, and minor amounts of benzene, toluene, methane, ethane, carbon monoxide, carbon dioxide, and various polymeric components. The effluent gas from the dehydrogenation reactor may be partially cooled in a waste heat exchanger or the like and then is typically fed to a condenser wherein styrene, unreacted ethylbenzene, divinylbenzene, sundry polymeric materials and the aqueous component are condensed with the hydrogen, methane, ethane, carbon monoxide and dioxide and the benzene and toluene remaining in the gas phase.
The partially condensed effluent is next fed to the phase separator wherein the gas phase is separated and subsequently treated to recover benzene and toluene. The aqueous phase is separated and, in most cases, is used as boiler feedwater. The hydrocarbon phase comprising styrene monomer and ethylbenzene is then subjected to distillation or other purification processes for separating styrene monomer from the ethylbenzene.
In the purification or distillation process stream, polymerization inhibitors are commonly added to inhibit undesired styrene monomer polymerization. Examples of addition of polymerization inhibitors at the purification stage include: U.S. Pat. No. 6,024,894 (Arhancet) wherein a combination of quinone methides and hydroxylamine are used; and U.S. Pat. No. 4,003,800 (Bacha et al.) wherein quinone alkides are charged to the purification zone.
In addition to the desire to inhibit polymerization in the purification stage, styrene monomer polymerization may also occur at process locations upstream thereof. For example, the styrene monomer present in the hot dehydrogenation effluent may polymerize causing fouling of the condenser and separator equipment. In many cases, polymerization treatments that are effective when injected in the purification process do not function effectively as inhibitors in the dehydrogenation section of the process. These known inhibitors sometimes partition to the aqueous phase and are therefore not available to inhibit hydrocarbon phase styrene monomer polymerization in dehydrogenator process lines and equipment such as the main condenser and phase separator. Additionally, some inhibitors maybe carried as contaminants in the aqueous phase and, as such, are undesirable components of the aqueous phase as it may ultimately be used as boiler or other process feedwater.